Apparatus and circuit for use with capacitive presence detection systems

ABSTRACT

A capacitance presence sensing apparatus ( 10 ) having a fail-safe circuit ( 13 ) and modular units ( 100 ). The fail-safe circuit ( 13 ) includes delivering a low voltage (FIG.  5 ) to a sensor and connecting voltage through a resistor to an oscillator ( 18 ). An opener ground in the circuit changes frequency of the oscillator ( 18 ) to indicate a problem With the fail-safe circuit ( 13 ). The apparatus also contains a number of inductance coils ( 28 ) which may be selected to tune the frequency of the system. Circuitry is provided in cards which may be combined together with receiver transmitter cards ( 106 ) and to easily facilitate the use of the system for desired applications.

FIELD OF THE INVENTION

The invention relates to an apparatus and circuits for use withcapacitive presence detection systems.

BACKGROUND OF THE INVENTION

Motion detection systems are known for detecting the presence of aperson or vehicle to a device such as a parking gate. One type of systemthat utilizes the rate of change in frequency of a signal from anoscillation from a continuous wave output is disclosed in U.S. Pat. No.5,337,039 to Simon. The system utilizes an oscillator which produces awave in a preset frequency range which is delivered to a capacitiveplate or sensor. The sensor is mounted to a device such as a parkinggate. The sensor is in the form of a coaxial cable which functions asone plate of a capacitor. The cable is attached by clips to the gate.When the sensor nears conductive objects, the frequency of the presetsignal changes. An error signal is generated if the rate of change infrequency exceeds a predetermined rate. The error signal is sent to acontrol unit which emits a signal to stop the movement of the gate.

However, presently known systems have practical limitations when placedin certain environments. Noise, environment and movement of the sensorcan limit the effectiveness of the system. It is thus an object of theinvention to provide a presence sensing system which may be simply andeasily used in a wide range of applications.

SUMMARY OF THE INVENTION

The capacitive presence sensing equipment of the present inventionincludes a fail-safe circuitry which is operative to provide a warningif the sensor is broken or damaged. A voltage is looped through thesensor and connected to a frequency oscillator, a DC short in thecircuit or low RF impedance path to ground results in the oscillatorproducing a low-frequency wave thereby generating a fail-safe signalwhen the frequency falls below a predetermined level. Thus, if thesensor is broken or damaged to produce a ground or short, a warningsignal is triggered.

The detection system also includes improved circuitry to preload thesensor with capacitance to reduce noise disturbance. The system is alsoprovided with a logic gate which prevents error signals from occurringwhen a sensor begins movement from a conductive object such a wall.Additionally, a steering diode is used to speed up a phase lock loopwhen the sensor is moving away from a capacitive object.

Additionally, the detection system is provided in modular componentswhich permits the system to be installed in any number of differentapplications such as on parking gates, moving sliding gates, garagedoors and security applications. The modular system accommodates bothhardwire and radio frequency transmission of data between the componentswhen the system is installed on moving objects. The components includean RS module, a com box, and a receiver box. The boxes accommodatedifferent circuit boards which may be inserted to permit customizing ofthe system for different applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent to those skilled in the arts upon reading the followingdescription with reference to the accompanying drawings in which:

FIG. 1 is a schematic of a capacitive presence sensing device having afail-safe circuit in accordance with the invention;

FIG. 2 is a schematic representation of a tunable LC tank circuit havinga plurality of inductors and capacitors made selectable through a DIPswitch;

FIG. 3 is a circuit diagram in accordance with the invention;

FIG. 4 is a perspective bottom view of a parking gate with a sensorinstalled for use with the fail-safe circuit in accordance with theinvention;

FIG. 5 is a perspective view of a slide gate having a modular capacitivepresence sensing system which is hardwired in accordance with invention;

FIG. 6 is a perspective view of a slide gate with capacitive presencesystem utilizing RF transmit signals in accordance with the invention;

FIG. 7 is a perspective rear view of a combox having three cards;

FIG. 8 is a perspective rear view of a combox adapted for use with onecard; and

FIG. 9 is a front view of a receiver box showing switches and warninglights and plugs in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a capacitive presence sensing apparatus 10 having anexternal sensor 12. As shown in FIG. 4, the sensor 12 may be mounted ona parking gate arm 68 to detect the presence of an object such as a caror person. The apparatus 10 has an oscillator 18 which generates apreset signal which is connected to the sensor 12 to provide anelectromagnetic field. When a conductive object enters the field, thefrequency of the preset signal changes. An error signal is sent when therate of change of the frequency is greater than a predetermined rate.When used in conjunction with a parking gate, the error signal is sentto a control unit to stop the movement of the gate. Although theapparatus 10 is disclosed for use with a parking gate, the apparatus 10may be used in a wide range of applications such as garage doors,security systems, etc.

In accordance with the invention, the capacitive presence sensingapparatus 10 includes a security and integrity fail-safe circuit 44 forthe sensor 12. A low voltage DC current (i.e. 12 volts) power source 15through a 470K pull up resistor 14 is delivered to one end of the sensor12 with the signal from an LC tank 16 connected to an a stablemulti-vibrator RC such as an oscillator 18. A suitable oscillator isproduced by Texas Instruments having a product number CD4060B. As shownin FIG. 4, both the DC current and pulsed signal are delivered to oneend of the sensor 12 through the center wire 70 of a coaxial cable. Theother end of the sensor 12 is connected through a resistor 20 such as a47K ohm resistor to the grounded shield 72 of the coaxial cable. Asshown in FIG. 1, the 47K resistor 20 completes the DC path to ground andholds the voltage terminal at pin “12” to logic low. A capacitor 27blocks the DC while providing a low impedance path for the continuouswave signal of the LC Tank Circuit 16. If the sensor 12 is damaged, suchas when a parking gate is broken, the circuit opens and no signal isproduced by the oscillator 18. The resistor 14 pulls up the DC level atterminal pin “12” to logic high thus inhibiting the oscillating signal.A DC short in the circuit or low RF impedance path to ground 22 resultsin the oscillator 18 producing a low frequency wave. As shown in FIG. 1,the oscillator 18 is connected to a failure to detect circuit 44 whichproduces a fail-safe signal when the frequency falls below apredetermined frequency level. Thus an open circuit or a low impedancecondition results in the frequency dropping below the frequency cut-offlevel, a fail-safe signal is generated to indicate a failure in thesystem.

As shown in FIG. 1, the presence detection system 10 also includesimproved circuitry to preload the sensor with capacitance to reducenoise disturbance.

As shown in FIGS. 1 and 2, the LC tank circuit 16 also includes a seriesof coils 28 having values which are used to change the value ofinductance in the LC portion to adjust the frequency of the system. Aset of dip switches 30 are provided to select the coil value. Differentantenna sensors have different capacitance, thus a coil value can beselected to keep in the desired band. By selecting the frequency of thesystem, the system can also be tuned to avoid disturbance from othernearby transmitters or other electromagnetic interference (EMI).

As shown in FIG. 1, the output from the RC oscillator 18 is divided by1024 and then passed to a phase lock loop circuit 32 (PLL) such asCD4046B made by Texas Instruments as well as one input of a three inputNOR gate 34. The output of the NOR gate 34 will only go high when allthree inputs are low. The output from the phase pulse pin of the phaselock loop 32 which represents a change in frequency is delivered as asecond input into the NOR gate 34. The PLL 32 has a frequency which is afactor of 1024 of the operating frequency of the LC 16.

When there is a downward shift in frequency of the output signal whichoccurs with an increase of capacitance at the sensor 12, such as whenapproaching an object, the output of the NOR gate 34 is a square wavepulse which represents rate of change in only a downshift in frequency.The PLL circuit 32 also includes a steering diode 38 to speed up the PLLwhen there are upward shifts in frequency to reduce the rate of changein the frequency which occurs when the sensor moves away from aconductive structure such as a wall.

In applications where the sensor is static, the elimination of thesteering diode and NOR gate 34 permits the system to “see” objectsmoving away or towards the sensor. Furthermore, by reversing thesteering diode and using a comparator output from the PLL in place ofthe 1024 from the LC on the input of the NOR gate 34, the system will“see” only objects moving away from the sensor.

As known in the art, the system includes a noise blank 36 that includesan EMI antenna to detect possible EMI interference. As best illustratedin FIG. 3, the output from the EMI antenna is coupled through atransistor and coupled as a third input signal to the NOR gate 34. Thus,whenever the EMI exceeds a preset threshold, the transistor turns onthus providing a high input to the NOR effectively disabling the NORgate 34 and providing an output to energize the opticoupler circuit 38speeding up the PLL.

After the EMI interference is removed, the transistor switches to a lowoutput and the opticoupler remains energized for a short period, i.e. 1–2 milliseconds to remove the residual effect of the disturbance in thePLL. An RC (not shown) is used to create this period thus providing atime delay before the PLL can produce an error signal indicating anobject detected by the sensor antenna.

As known in the art, during normal operation, i.e. EMI below thethreshold, the output from the NOR gate 34 is fed through a sensitivitycontrol 40. The sensitivity control 40 comprises a series of differentcapacitors which are selectably electrically connected with the outputfrom the sensitivity control. The selected capacitor in the rangecontrol charges whenever the output from the NOR gate is high inaccordance with the RC time constant of the selected capacitor andsensitivity control.

The output from the sensitivity control 40 is fed as the input triggersignal to a Schmidt trigger 42. Whenever the voltage from the output ofthe sensitivity control as controlled by the charging capacitor in therange control exceeds a predetermined voltage threshold, the Schmidttrigger 42 is activated thus switching its output signal Q to a highstate. The output signal Q turns on an FET switch 48 which in turnactivates a first relay (not shown). This relay can be used, forexample, to reverse the direction of travel of the automotive swinggate.

The output signal Q from the Schmidt trigger 42 also may be coupledthrough a latch delay circuit and ultimately coupled as an input signalto a second FET. This FET is thus activated at a predetermined timeperiod, e.g. 6–35 seconds, after the activation of the first FET. Areset disable circuit is also coupled to selectively disable the Schmidttrigger circuit curing an initial power on condition. The reset disablecircuit prevents false trigger of the FET during a power on condition.

A failure detect circuit 44 is also preferably connected to the outputfrom the oscillator to provide an alarm 46 whenever the entire system isdisabled by the fail-safe circuit.

As shown in FIGS. 5–9, an improved modular presence detection system 100is adapted for use with a slide gate system 80. The modular systempermits use of components that permit the system to be easily adaptedfor a large number of applications. The modular system 100 may be usedwith any presence sensing application. The slide gate system 80 has agate 82 which is moved reciprocally between pairs of protection posts 84by a drive unit 102 (not shown) and controller 94. The modular unitsinclude an RS box 86, a com box 88 and a receiver box 90. Each sensor 92is connected to one RS box 86 which is mounted near one end of thesensor. In the system 80 shown in FIG. 6, each end of the gate 82 andthe posts 84 are provided with a sensor 92 and RS box 86. The RS box 86includes the oscillator and fail-safe circuit. The com box 88 is mountedto the gate 82 and receives a signal from the RS box 86 and. sensor 92.The RS box 86 produces signals which are delivered to the receiver box90 located in or adjacent to a structure system controller 94. Thereceiver box 90 receives the signals by wire 96 as shown in FIG. 5 or bya radio frequency transmitter installed in the com box 88 as shown inFIG. 6. The com box 88 delivers a signal such as “stop”, “stop andreverse” or “fail safe”. The controller 94 then directs the drive unitto the appropriate action.

The com box 88 contains from one to three cards, three cards shown inFIG. 7 or one card as shown in FIG. 8. The cards may be either a RSCcard 104 and a radio transmitter card 106. The com box 88 includes apower source, such as a lithium battery and may include a solar cell orother charging means to charge the battery. One RSC card 104 isnecessary for each capacitive sensor 12 and RS box. The RSC includes theelectronic circuitry necessary to produce an error signal or fail-safesignal. The com box 88 includes both a bus 108 and a port 110. The bus108 permits two-way connection of all cards in the com box 88. The combox 88 may contain one to three RSC cards 104. Alternatively, the combox 88 may contain one or two RSC cards 104 and one radio transmittercard 106.

The radio transmitter card 106 is connected to the power source andtransmits signals received from any of the RSC cards 104 to the receiverbox 90. The radio transmitter card 106 permits the com box 88 to be usedon moving structures such as slide gates or in remote locations. Someapplications could require four or more sensors mounted to a particularstructure. Each sensor 92 is connected to the corresponding RSC card 104in the com box 88. In some applications, one or more of the RSCs in acom box may be connected through the ports to other communicationdevices such as a telephone transmitter, equipment controller or alarmsystem.

The receiver box 90 includes a power source, a relay system and slots toreceive either the RSC card 104 or a radio receiver card 106. Thereceiver box 90 is typically located in the control box for themechanism such as a parking gate, slide gate, or door opener. However,can be used in security system or other applications. The radio receivercard is tuned to receive signals broadcast by the radio transmitter card106 in the com box 88. The signals are either error signals or fail-safesignals. The signals are delivered to a relay which diverts theappropriate signal to the logic system of the apparatus. Error signalsare converted to logical signals such as stop, stop and reverse, orreverse which are then delivered to the equipment controller.

The receiver box 90 also contains lights 114 for monitoring thecondition of the system. The receiver box 90 also contains switches (notshown) for selecting the receive frequency as discussed above. Finally,the box has connections to an RS box, a com box, or a control box 94 todeliver and receive the appropriate signal.

1. A capacitive presence sensing system comprising: a sensor; anoscillator having a output connected to said sensor; said presencesensing system having a fail-safe device, said device comprising: asubstantially constant DC voltage source connected by an electricalcircuit through said sensor; and signaling means connected to saidvoltage source, said signaling means operable to generate a signal whensaid electrical circuit is open.
 2. The capacitive presence sensingsystem of claim 1 wherein said signaling means further comprises anoscillator, said oscillator responsive to a change in voltage defined bysaid electrical circuit to change a frequency of a signal generated bysaid oscillator.
 3. The capacitive presence sensing system of claim 1wherein said electrical circuit further comprises a coaxial cable havinga center wire and a shield, said center wire being connected to saidvoltage source and said shield being connected to said signaling means.